Strange problem with Time To First Byte

When performing various speed tests in the previous post, Secure site / https, SSL (Speed tests on site B – Part 4), I ran into a strange problem that I couldnt solve by myself. The Time To First Byte was suddenly and consistently taking around 4s, a behavior I had not seen ever before.

I flushed all the caches one by one, then turned them off. Still no difference.

Then I reached out to the excellent SiteGround support, and they immediately gave me a solution. They renamed the object-cache.php file under wp-content, and things were back to normal after that!